The invention is a wind turbine rotor with a vertical rotation axis, used in turbines applying the Darrieus' principle while processing wind energy into mechanical energy from the movement of rotation.
Solutions of turbine rotors with a vertical rotation axis are known. For example, U.S. Pat. Nos. 4,264,279 and 4,430,044 have at least two horizontal supports with vertically mounted rotor blades on the ends that are connected with the hub of the drive shaft. Rotor blades have mostly a symmetrical airfoil and are connected with a support in the center zone in relation to their height, with the division of the blade into an upper wing and a lower wing. During rotor movement exerted by wind pressure, there is both a driving aerodynamic force and a centrifugal force. Both forces act in the centers of the mass of the two wings. In the middle of the trajectory of the blade rotation movement, the aforementioned forces are directed at the same direction—resulting in longitudinal deformation of the wings and high bending stress in the connection zone of the blade and support. On the other half of the circle, on the side of the wind direction, both of these forces are in reverse.
The pulsating load changes, including polarity changes, that appear have a significantly adverse effect on turbine wear and equipment efficiency. Known solutions consist of inserting additional elements into the construction of the rotor in order to stiffen the wings. For example, British Patent GB 2175350t describes line tie rods like the rotors, or additional supports, disclosed in the German Patent DE 3626917. The introduction of the strengthening elements results in an increase in aerodynamic drag and a decrease of turbine efficiency, especially if they include acutely angled forms that facilitate volumetric turbulence. The adverse effect of the centrifugal force leads to a partial reduction in the application of the changeable airfoil on the rotor length, with the length of the chord and the thickness of the airfoil decreasing towards both wing ends. Such solution, among others, was applied in the rotor disclosed in the patent description of EP 0046370. In the brief description of the working conditions and technical problems appearing in these kinds of rotors, it is necessary to indicate the diversity of aerodynamic force on the upper and lower wings and the construction susceptibility to self-excited aeroelastic vibrations (i.e., the flutter of wings at high speeds of the surrounding air). The characteristics of the area may often determine whether there is a substantial decrease of wind speed at the lower wing level.
The aim of this invention is to draw up a simple construction of a rotor, characterised by high stiffness and durability, low aerodynamic drag, and high efficiency of wind pressure transformation on the driving force of the rotor shaft.
In this invention, like to the above disclosed solutions, the rotor involves at least two horizontal supports connected to the hub. Rotor blades (each consist of two wings joined together) with a symmetric or concave-convex airfoil, with chord lengths and airfoil thickness decreasing toward both wing ends, are fixed tightly to the ends of the supports. The essence of the invention is that the upper and lower wings of the rotor blade deflect from the central zone radially outward at the angle relative to the axis of rotation. At the same time, the chord lengths of the airfoil at both wings' ends and the chord length in the central zone are approximately inversely proportional to the radii of their location relative to the axis of rotation.
The use of a rotor with deflected wings whose airfoil decreases in chord and thickness towards their span from center to tips, provides a stable intensity of wind power consumption along the length of the wings. Altering the wing airfoil chord and thickness, by moving the center of the wings mass closer to the center of the blade, decreases the bending moment caused by centrifugal forces directed at the wings which cause their deformation. The frequency of free blade vibrations is higher with deflected wings whose airfoil decreases in chord and thickness than in the case of straight blades. This facilitating result is particularly visible during gust winds. The deflected wings introduce an aerodynamical twist, that is, the angle of incoming air flow near the central zone of the wing is bigger than at the wings ends. The diversity of angle attack, in practice, eliminates the danger of flutter.
Further embodiments of the invention aim to eliminate the influence of different wind speeds that appear at the upper and lower levels of the rotor. To that end, the angle of deflection of the lower wing in the rotor should be larger than the angle of deflection of the upper wing. The advised angle difference is in the range of 1° to 5°.
The solution in which the lower wing is longer than the upper one is also beneficial. The recommended length difference is in the range of 2% to 15%.
In accordance with the invention, it would be advisable to use supports in the rotor with a symmetrical airfoil and horizontally placed chords, as well as a connection with the hub such that the longitudinal axes goes through the geometrical center of the airfoils, and intersects the axis of rotation.
The rotor works most efficiently when attacked by airflow at optimal angles, it is also useful to connect the driving blades to the supports using known set points of attack angles, which enable regulation in the range of −2° to +3°.